1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device which has a current detecting function.
2. Description of the Related Art
A multi-base/multi-emitter type transistor T as shown in FIG. 6 has been proposed as a transistor with a current detecting function (see Japanese Unexamined Patent Publication No. 3-008369). The transistor T has a common collector C. The transistor T has two separate bases, a main base B and a sense base B.sub.S, and two separate emitters, a main emitter E and a sense emitter E.sub.S. A load 21 is connected to the collector of the transistor T. Constant currents I.sub.B and I.sub.BS in a predetermined ratio are input respectively to the main base B and sense base B.sub.S from a drive circuit 22. When a sense current I.sub.ES flows across a sense resistor R.sub.S, the value of the sense current I.sub.ES is acquired by measuring a voltage at both ends of the sense resistor R.sub.S. Based on that value, a collector current I.sub.C is detected. A main emitter current I.sub.E and the sense current I.sub.ES in a predetermined ratio run respectively through the main emitter E and the sense emitter E.sub.S without the influence of either the collector current I.sub.C or changes in the ambient temperature. It is therefore possible to detect the current with high accuracy.
The conventional transistor with a current detecting function requires that constant currents (I.sub.B and I.sub.BS) in a predetermined ratio should be input respectively to the main base B and sense base B.sub.S. This inevitably complicates the structure of the drive circuit 22. Further, the constant currents, particularly, the main base current I.sub.B flowing through the main base B is great, so that the power loss in the drive circuit 22 will increase.